This invention relates to apparatus for elevating a truck body particularly for trucks which are designed primarily but not solely for the transportation and unloading of grain, gravel and other comminuted material. Such apparatus is generally known as a truck hoist and can be used on trucks, semi-trailers or farm trucks.
Conventional truck hoists as commercially manufactured and used, fall into two groups. In the first group, a telescopic ram assembly extends between the front of the truck body and the frame or chassis of the truck which, when elevated, tilts the truck body around a rear pivot thus permitting the material to slide from the truck body. In order to achieve sufficient elevation of the body relative to the chassis, the telescopic ram assembly must include a number of stages. Multi-cylinder telescopic type lifting devices of this type are relatively expensive and can require considerable modification to the truck frame and to the truck body. Such multi-stage cylinders are only single acting, thus allowing the box to rattle on the frame when travelling over rough roads with the box empty.
In a second group, a scissors-type hoist is used, situated between the frame of the truck and engaging the truck body approximately half way along. A disadvantage of this conventional scissors-type is that it requires considerable modification of the frame and a heavy duty structure, all of which add considerable costs to the installation. The scissors-type lift is also difficult to install without interfering with the drive shaft of the truck.
In both cases, the hoist lifts from a single point so that lateral stability of the body relative to the chassis is poor when it is being lifted.
Attempts have been made to overcome these disadvantages in apparatus disclosed in a number of patent specifications. Particularly in U.S. Pat. No. 2,988,399 there is disclosed an apparatus in which the hydraulic ram comprises only a single stage which lifts the truck body to a first partly elevated position. Following this partial elevation, the body is further elevated by a complex arrangement in which the spacing between the rear axles and the ram is reduced thus acting to lever the body upwardly relative to the rear axles. This solution is extremely complex and requires considerable further modification to the truck chassis thus completely overcoming any advantage which may be obtained by replacing the conventionally used equipment.
In a second prior arrangement disclosed in Japanese laid open application No. 55-148626, a first multi-stage hydraulic cylinder arrangement is supplemented by the use of a scissors-type hydraulic lifting arrangement. Again this apparatus is totally unsatisfactory in that it incorporates the disadvantages of both the conventional types of apparatus.